In the prior art, identification systems are known which include one or more stationary read/write devices which exchange data in a contactless manner with mobile data memories via a data transmission link which usually is radio-based. Systems of this type are used in technical installations in which a large number of objects or commodities must be moved as quickly and freely as possible. The objects may be of various types, e.g. packages in a dispatch installation, assembly parts in a manufacturing plant, items of baggage in a transportation system, etc.
An example of an identification system of this kind is described in the ISO-18000-4-MOD3 standard entitled “Radio-Frequency Identification Standard for Item Management—Air Interface”.
The standard provides that polling for the presence of a mobile data memory in the detection range is carried out by the read/write device. For this purpose the read/write device emits an unmodulated carrier signal with a predefinable backscatter frequency of, for example, 2.45 GHz. The signal can be returned to the read/write device passively, e.g. by backscattering, by a mobile data memory located within the reception range.
Independently thereof, the mobile data memory modulates the impedance of an integrated transmit/receive antenna in cyclical sequences with a significant recognition sequence to identify the mobile data memory to a read/write device. In addition, the read/write device receives time information as to when the mobile data memory will switch on its data receiver. If the read/write device can receive the returned modulated backscatter frequency, the validity of the reply is checked. If it is found to be valid, the read/write device applies a data modulated carrier signal at the time when the mobile data memory is expected to be ready for receiving. This signal contains, firstly, the unmodulated backscatter frequency and, secondly, a data modulated communication frequency which signals to the mobile data memory that a data transmission will now follow. The transmitted data may contain, for example, an identification number of the read/write device. By means of this identification number it can be determined, for example, whether the data awaiting transmission is intended for the mobile data memory or not. For the mobile data memory this means that valid or invalid data is to be sent. The mobile data memory can also classify as invalid data already transmitted during a data reception, if inconsistencies, e.g. through transmission errors, are recognized.
A signal received by the receive antenna of the mobile data memory is now polled cyclically and at short intervals for a presence of the data modulated carrier signal, i.e. for a simultaneous presence of the backscatter frequency and the data modulated communication frequency. If both frequencies are detected, the data receiver of the mobile data memory remains switched on to receive data.
For its power supply the mobile data memory usually has an energy store, in particular a battery. To achieve the longest possible operating life, therefore, it is necessary to minimize the power consumption. A known measure for reducing power consumption is, for example, to select especially power-saving electronic components. In addition, the circuit design of a mobile data memory should take account of the fact that many times more current is required to receive data than to transmit data. Unlike the very low power requirement for the passive backscattering described in the introduction, for which the antenna impedance is modulated only momentarily, several circuits must be connected for data reception. In addition, these circuits, such as data demodulator, amplifier, mixer, etc., require a minimum time until transients have subsided and the components are ready for operation. Moreover, account should be taken of the fact that in operational use data is generally transmitted between read/write device and mobile data memory only for fractions of the total operating time. It is therefore recommended in the above-mentioned standard that the data receiver of the mobile data memory be switched on only cyclically and for a short period to reduce power consumption.
To achieve data reception at a data rate of 384 Kbit/s according to the above-mentioned standard, at least double the reception clock frequency of 384 kHz is required to be able to sample a received signal before demodulation. Frequently, however, oversampling of the incoming signal is essential in order to achieve a more reliable reading of the data. In that case a possible clock frequency would be, for example, four or eight times the above-mentioned required data rate. This would correspond to a clock frequency of 1536 MHz or 3072 MHz respectively. From this clock frequency the data rate of 76.8 Kbit/s laid down for cyclical transmission operation by the standard can be derived by means of dividers connected to the output. But even with the exemplary clock frequency of 1538 MHz, which still yields satisfactory values for data reception, the mean power consumption of the associated quartz oscillator is approximately 20 to 30 μA.
Despite all the aforementioned measures it has not been possible up to now to minimize power consumption to the extent that a battery exchange during the life of the mobile data memory can be dispensed with. This has the associated disadvantage that with an exhausted battery the data stored in the mobile data memory can be lost.
A further disadvantage is that to exchange the battery in time, the mobile data memory must, for example, be withdrawn from the production process. This can cause delays and interference in the process flow. Depending on the case and the field of application, the required life of a mobile data memory can be approximately 10 to 15 years.